Oxazoline surfactant anode additive for alkaline electrochemical cells

ABSTRACT

According to the present invention an alkaline electrochemical cell can contain an anode having an anode active material, an alkaline electrolyte, a gelling agent and an oxazoline surfactant additive. The invention relates to an anode mix, to an anode containing the mix, and to an electrochemical cell containing the anode and to methods for making the anode mix, the anode and the cell. Performance improvements can be realized when the oxazoline surfactant is provided in the anode, which can include increased operating voltage, good high rate pulse capability, elimination of initial potential dip, good shelf life and reduced sensitivity to open circuit rest.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The demand for very small electrochemical cells has increasedwith increased use of small, electrically-powered devices, and alkalineand alkaline metal-air electrochemical cells are an increasingly popularchoice for powering such devices as hearing appliances and computers.Metal air cells contain an anode and an oxygen cathode, with the oxygenusually obtained from ambient air. The oxygen cathode catalyticallypromotes the reaction of oxygen with an aqueous neutral or alkalineelectrolyte and is not consumed or changed during discharge. As thecathode is extremely compact yet has essentially unlimited capacity,very high energy densities are achieved, resulting from the increasedavailable volume for the anode active material. Consequently, ametal-air cell can provide more watt-hours of electromotive force than aso-called “two-electrode cell” of similar cell size, mass and anodecomposition that contains both anode- and cathode-active materialsinside the cell structure. Metal-air cells have an advantageouswatt-hour capacity/mass ratio without regard to size or configuration,which can be, e.g., button cells as in, for example, U.S. Pat. No.5,721,065 (“the '065 patent”), or cylindrical cells as in, for example,U.S. Pat. No. 6,210,827 (the '827 patent”), each incorporated byreference in its entirety as if set forth herein.

[0004] A typical metal air cell such as a zinc air cell contains ananode that includes a zinc alloy powder, a carbon-based air cathode andan alkaline electrolyte with a gelling agent to prevent the electrolytefrom creeping through the cell seals. For efficient operation, the anodeof a zinc air cell should participate only during current-generatingreaction periods. In reality, though, corrosive shelf-discharge sidereactions in the alkaline electrolyte can reduce both service and shelflife of alkaline electrochemical systems that use zinc as the anodeactive material. Much effort has been directed to improvingelectrochemical reaction efficiency and cell output. Previously, mercurywas added to cell anodes to improve corrosion resistance. In recentyears, mercury has been replaced by substances that conform toenvironmental requirements. Small amounts of metals such as lead,indium, and bismuth, and combinations thereof, can effectively improvecorrosion behavior of anodic zinc. Certain organic surfactants canprovide effective corrosion-inhibiting effect at the metal surface whilemaking the anode sufficiently available for electrochemical oxidationthat the cell output can be maintained under heavy cell loading.Surfactants can, like mercury, improve discharge capacity, service lifeand shelf life, without substantial adverse environmental impact. U.S.Pat. No. 4,857,424 (“the '424 patent”) incorporated by reference hereinas if set forth in its entirety, discloses reduced-mercury ormercury-free zinc-manganese dioxide cells containing an organosiliconatetype surfactant. Also, the '065 patent discloses a button cell having ananode mix containing zinc metal powder, indium or other compounds,optionally a low level of mercury, a gelling agent and preferably ahydroxyethylcellulose surfactant. The anode disclosed therein sustainslonger periods of power production at a relative steady voltage of atleast 1.1 volts while protecting the anode metal from corrosion in thealkaline environment.

[0005] Similarly, the '827 patent discloses a cylindrical cell having ananode mix containing electrolyte, a gelling agent, particulate zinc,zinc oxide, additives and an organic surfactant comprisinghydroxyethylcellulose.

[0006] Japanese Patent No. JP10083812 to Toshiba Battery Co. Ltd.discloses providing a high performance zinc alkaline battery containing0.5-100 ppm of a fluorine-containing surfactant in anon-amalgamated-zinc-alloy-based gelled anode having a viscosity of100,000 to 300,000 cPs at 25° C.

[0007] U.S. Pat. No. 5,382,482 concerns suppressing dendrites and shapechange in an alkaline cell by disposing a cross-linked polymer filmlayer in close proximity to the anode active material, where the polymerfilm layer can contain, among other polymers, polyoxazoline. The patentdoes not describe including an oxazoline surfactant in the anode mix ofthe cell.

[0008] Further improvements in performance, such as increased operatingvoltage, improved discharge profile and reduced sensitivity to opencircuit rest in alkaline electrochemical cells, preferably withoutsacrificing known benefits, are welcome and desired.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention discloses that an alkaline electrochemicalcell having superior discharge performance and corrosion resistanceincludes an oxazoline surfactant additive in the cell anode. In oneaspect, the present invention relates to an anode for use in a alkalineelectrochemical cell, where the anode contains a metal anode activematerial, an anode electrolyte and an oxazoline surfactant. Optionally,the anode of the invention can include additional components known inthe manufacture of alkaline electrochemical cells and anodes for same.In a related aspect, the invention relates to an alkalineelectrochemical cell containing an anode of the invention. In anotheraspect, the present invention relates to methods for making an anode ofthe invention, or for making a cell containing the anode, as aredetailed below.

[0010] It is an object of the invention to improve discharge performanceand corrosion resistance without sacrificing the known benefits ofadding surfactants to alkaline cell anodes.

[0011] It is a feature of the invention that the anode, and anelectrochemical cell containing the anode, contain an oxazolinesurfactant that can coat at least a portion of the particles of theanode active material.

[0012] It is an advantage of the present invention can maintain knownbenefits of anode surfactants, and can provide additional benefits thatcan include improved discharge efficiency at low and high rates, highoperating voltage, lack of initial potential dip, and reducedsensitivity to open circuit rest.

[0013] Other objects, features and advantages of the present inventionwill become apparent in view of the following detailed description ofthe invention and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 compares the discharge profiles of cells of the inventioncontaining an oxazoline surfactant and cells containing anorganosiliconate surfactant. Discharge was at 374 Ohm for 16 hours/dayat 70° F., 50 % relative humidity, after one month storage at 70° F.

[0015]FIG. 2 compares the discharge profiles of cells of the inventioncontaining an oxazoline surfactant and cells containing anorganosiliconate surfactant. Discharge was at 620 Ohm for 16 hours/dayat 70° F. 50 % relative humidity, after 20 days storage at 140° F.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention relates to using an oxazoline-typesurfactant, which can be a fatty oxazoline surfactant, as an additivefor an alkaline electrochemical cell anode. Cells comprising the anodecan exhibit improved electrical discharge performance, including higheroperating voltage, good high rate pulse capability, no initial potentialdip and reduced sensitivity to open circuit rest in comparison to cellsthat use other surfactants.

[0017] Oxazoline-type surfactants can be added to anode materials of anyalkaline metal-anode electrochemical cells to improve dischargeperformance and to improve discharge capacity, service-life, and shelflife. While the invention was exemplified with metal-air anodes andcells, more particularly in zinc-air cells, the invention can beemployed in metal-containing anodes generally, more particularly in zincanodes, and in cells containing same. Suitable chemistries include, butare not limited to, zinc-air, zinc-manganese dioxide, zinc/AgO andzinc/Ag₂O systems, without regard to size or configuration. Preferablecells are of standard configurations, such as button cells, cylindricalcells and prismatic cells, which are available in a variety of standardsizes. For example, oxazoline type-surfactants can be substituted forthe surfactants described in incorporated U.S. Pat. Nos. 4,857,424,5,721,065 and 6,210,827. The anode materials to which an oxazolinesurfactant is added contain a metal powder, a gelling agent, and anelectrolyte. The anode materials can also include other componentsincluding but not limited to mercury, indium, gallium, aluminum, lead,bismuth and calcium compounds, and combinations thereof, as disclosed,for example, in the '065 patent and in U.S. Pat. No. 5,721,072, alsoincorporated herein by reference as if set forth in its entirety.

[0018] In one aspect, the present invention relates to an anode mix thatincludes a metal powder suitable for an anode of an alkalineelectrochemical cell, an electrolyte compatible with the anode metal, anoxazoline surfactant and a gelling agent. In another aspect, the presentinvention relates to an anode that includes the composition describedabove in a container for receiving the composition of matter. In stillanother aspect, the present invention relates to an alkalineelectrochemical cell that contains the anode. The cell has a dischargevoltage of at least 0.7 volts, preferably 1.0 volts, more preferably 1.1volts and still more preferably 1.2 volts, during its service life.

[0019] In preferred embodiments, the metal powder is preferably about55% to about 73% by weight of the anode, the electrolyte is about 27% toabout 45% (by weight) in an aqueous solution, and the oxazolinesurfactant is about 0.001% to about 5% (by weight), based on the weightof the particles of metal powder. The anode material generally alsocontains about 0.1% to about 0.5% of gelling agent, by weight based onthe weight of active material, and between about 1% to about 4%,preferably about 2% of zinc oxide, by weight based on the weight of theelectrolyte. In more preferred embodiments, the metal powder is aparticulate zinc alloy powder and the electrolyte is potassiumhydroxide.

[0020] In a related aspect, the present invention is a method of makingan anode mix for use in the anode and in the cells of the invention, themethod involving the steps of mixing a metal powder, a gelling agent, anoxazoline surfactant, and an electrolyte in the above-mentionedproportions. The order in which the components are combined is notcritical, except insofar as when mixing the oxazoline surfactant with aliquid (namely, the electrolyte or the gelled electrolyte), thesurfactant is preferably a state (e.g., melted). Alternatively, if thesurfactant is mixed first with the metal powder or other dry component,it can be a solid. In one embodiment of the method, the gelling agentand the alkaline electrolyte are combined first to form a gelledelectrolyte. Then, the oxazoline surfactant is added to the gelledelectrolyte with agitation. Finally, the metal powder is combined withthe surfactant-containing alkaline electrolyte to form the gelled zincanode material which can be incorporated into the receiving container.

[0021] In another related aspect, the present invention is a method ofmaking an alkaline electrochemical cell involving the step ofincorporating the aforementioned inventive anode into an electrochemicalcell, the process for making the cell being, apart from the compositionof the anode, conventional as is shown, for example in U.S. Pat. No.4,617,242, incorporated herein by reference as if set forth in itsentirety.

[0022] A suitable oxazoline surfactant can be solubilized in ananode-compatible electrolyte and is a liquid or can be liquefied underthe anode processing conditions. U.S. Pat. No. 3,389,145, incorporatedby reference herein as if set forth in its entirety, disclosesstructures of one suitable set of oxazolines and processes for makingsame. Also suitable for use in the anode of the invention aresubstituted oxazoline surfactants having the structures shown in U.S.Pat. No. 3,336,145, in U.S. Pat. No. 4,536,300, in U.S. Pat. No.5,758,374 and in U.S. Pat. No. 5,407,500, each incorporated by referenceherein as if set forth in its entirety, and mixtures of any of theforegoing. A most preferred oxazoline surfactant, ethanol,2,2′-[(2-heptadecyl-4(5H)-oxazolydine) bis(methyleneoxy-2,1-ethanedyloxy]bis, has a structure shown as Formula(I-2) in incorporated U.S. Pat. No. 5,407,500. This compound iscommercially available from Angus Chemical (Northbrook, Ill.) asAlkaterge T-IV.

EXAMPLE

[0023] Zinc air cells according to the invention were built with ananode as described in incorporated U.S. Pat. No. 4,617,242. except thatthe oxazoline surfactant was substituted at 0.3%, relative to thealkaline electrolyte in test cells in place of the organosiliconatesurfactant of the prior anode composition. The structure of the test andcontrol cells was as described in incorporated U.S. Pat. No. 5,721,065.

[0024] The discharge characteristics of 13 size zinc air cells of thepresent invention containing Alkaterge T-IV were compared againstotherwise identical commercial cells having an organosiliconatesurfactant in the anode. FIG. 1 illustrates the discharge curve at 374Ohm after 1 month storage of cells at 70° F.; FIG. 2 shows the dischargecurve at 620 Ohm after 20 days storage at 140° F. As shown in FIGS. 1and 2, cells containing oxazoline-type surfactant-based exhibitedoperating voltage at least 5-10 mV higher than prior art cells duringmost of their usable lives. In addition, the test cells did not exhibitan initial potential dip and were less sensitive to open circuit restthan otherwise comparable cells containing the organosiliconatetype-surfactant.

[0025] Further, Table 1 shows discharge capacity values at 374 Ohm oftest cells containing Alkaterge T-IV surfactant. The capacity of thesecells was comparable to that of organosiliconate type surfactant-basedcells before and after storage for one month at 70° F., 50% relativehumidity, although discharge capacity before storage was comparativelylower in cells that contained Alkaterge T-IV. TABLE 1 Discharge Capacity(mAh) at 1.1 Volt Control Alkaterge T-IV Alkaterge T Alkaterge E NoDelay 251 244 247 N/A One month 252 252 198 N/A

[0026] Additionally, cells containing Alkaterge T-IV and control cellswere also stored for twenty days at 140° F., 50% relative humidity andwere tested at 620 Ohm for 16 hours/day at 70° F., 50% relativehumidity. Results of the two cell types were comparable (258 mAh v. 254mAh, respectively).

[0027] Taken together these results demonstrate that oxazoline-typesurfactants can effectively maintain cell capacity. Also, oxazoline-typesurfactant based cells exhibited good high rate pulse capability ascompared to the control cells.

I claim:
 1. An anode mix comprising an alkaline electrolyte, an anodeactive material, a gelling agent and an oxazoline surfactant.
 2. Theanode mix of claim 1 wherein the oxazoline surfactant comprises a fattyside chain.
 3. The anode mix of claim 2 wherein the oxazoline surfactantis ethanol, 2,2′-[(2-heptadecyl-4(5H)-oxazolydine) bis (methyleneoxy-2,1-ethanedyloxy]bis.
 4. The anode mix of claim 2 wherein the oxazolinesurfactant comprising the fatty side chain is Alkaterge T-IV.
 5. Theanode mix of claim 1, wherein the alkaline electrolyte is potassiumhydroxide.
 6. The anode mix of claim 1, wherein the anode activematerial is particulate zinc powder.
 7. The anode mix of claim 1,wherein the surfactant coats at least a portion of the anode activematerial.
 8. An anode for use in an electrochemical cell, the anodecomprising the anode mix of any of claims 1-7.
 9. An alkalineelectrochemical cell comprising a cathode, an anode, and an alkalineelectrolyte in electrical contact with the anode and the cathode, theanode comprising an anode active material, a gelling agent and anoxazoline surfactant.
 10. The alkaline electrochemical cell of claim 9wherein the oxazoline surfactant comprises a fatty side chain.
 11. Thealkaline electrochemical cell of claim 10 wherein the oxazolinesurfactant is ethanol, 2,2′-[(2-heptadecyl-4(5H)-oxazolydine) bis(methyleneoxy-2,1-ethanedyloxy]bis.
 12. The alkaline electrochemicalcell of claim 10 wherein the oxazoline surfactant comprising the fattyside chain is Alkaterge T-IV.
 13. The alkaline electrochemical cell ofclaim 9 wherein the alkaline electrolyte is potassium hydroxide.
 14. Thealkaline electrochemical cell of claim 9 wherein the anode activematerial is particulate zinc powder.
 15. The alkaline electrochemicalcell of claim 9 wherein the surfactant coats at least a portion of theanode active material.